This section provides background information related to the present disclosure which is not necessarily prior art.
Core/shell nanoparticles may be used in a variety of applications, such as in the formation of an electrode of a battery. The core may be any type of powder particle. The shell may be a thin coating, which may be applied by a process such as chemical vapor deposition (CVD) or atomic layer deposition (ALD). In CVD, a mixture of gases flows around heated substrate particles to form a thin solid film to grow on the surface of particles by heterogeneous reaction. Byproducts are desorbed and pumped away. ALD, a variation of CVD, is a self-limiting process for chemical deposition or growth of ultrathin films on a substrate. ALD typically involves subjecting the target substrate particles to self-saturating surface reactions with a single vapor of precursor materials or multiple vapors introduced sequentially having the precursors of the surface coating. Each surface reaction results in a film that is one atomic layer thick. The surface reactions may be conducted sequentially and/or in an alternating fashion, depending on the composition and structure of the coating or film desired.
However, where the substrate is a powder or solid particles, the particles often clump together during the ALD and/or CVD process. Such clumping blocks active sites on the substrate particles and prevents adequate reaction with the precursors, resulting in particles that are not well coated; for example, on average only about 40% may be coated.
In fluidized bed reactors, a portion of the substrate particles settles to the bottom of the reactor and clumps together, thereby resulting in a low coating efficiency. Moreover, fluidized bed reactors are easily clogged and require a large amount of carrier gas. Alternatively, in rotary powder coaters, the substrate particles are contained in a small rotary tube and tend to clump together, reducing coating efficiency as described above. Rotary powder coaters are also susceptible to clogging and are uneconomical on a large scale. Cascade coaters maybe used to semi-continuously coat substrate powders. Cascade coaters include a series of vertically-connected reactors. Each reactor consists of a powder reservoir located above a precursor reservoir, where the two reservoirs are separated by a valve. The valve is opened to fluidize the powder substrate and achieve vapor deposition of a precursor onto the surface of the powder substrate. Each reactor may have independent pressure and temperature control. Coated substrate powder may be transferred down into subsequent reactors in the cascade coaters for exposure to additional precursors. However, the number of reactors and coating steps is limited due to space constraints. Mechanical failure of cascade coating systems is common because of its mechanical complexity and the need for a high degree of synchronization. Similar to the reactors described above, substrate powder in cascade coaters is subject to clumping. It would be desirable to develop a reactor for ALD and CVD that facilitates greater separation of substrate particles for more efficient coating.